The invention relates to a television camera circuit comprising a gamma and an aperture correction circuit for a television picture signal, the aperture correction circuit being provided with a correction signal generator for deriving, from the picture signal varying between a black level and a peak white value, an aperture correction signal having a negative and a positive-going signal value relative to a reference value.
Such a television camera circuit is proposed in U.S. Pat. No. 3,919,470. It is described that the combination of horizontal and/or vertical aperture correction, for emphasizing signal transition on a display, and gamma connection, for adaptation of the linear optoelectronic recording characteristic at the camera to the non-linear electro-optical display characteristic at the picture display apparatus, may result, with certain luminance values, in a deterioration instead of rather than an improvement in a picture quality. Namely, the gamma correction results in lower luminance values, at the black level in particular, in an exaggerated, excessively wide and, consequently, annoying emphasizing of signal transitions.
The above-mentioned patent discloses as a solution, the formation of an asymmetrical aperture correction signal to be added to the picture signal, the asymmetry in the correction signal being such that the subsequent gamma correction, effected on the aperture-corrected picture signal, does not furnish the exaggerated emphasis on the picture display. For completeness it should be noted that the degree of asymmetry has further been made dependent on the instantaneous value of the picture signal.